The present invention concerns a new process for obtaining ceric oxide having a large specific surface area at high temperature.
In the following description of the invention, by "specific surface area" is meant the B.E.T. specific surface area, determined nitrogen adsorption in accordance with the ASTM D 3663-78 standard as established based on the BRAUNER-EMMETT-TELLER method described in The Journal of American Society, 60, 309 (1938).
It is well known that ceric oxide may be used as a catalyst or catalyst carrier. Mention may be made, for example, of the work of Paul MERIAUDEAU and his colleagues, concerning methanol synthesis, taking CO+H.sub.2 as a basis, on catalysts on a plate set on ceric oxide (Reports of the Academy of Sciences, Paris, volume 297 - Series II-471, 1983).
It is also well known that the effectiveness of a catalyst generally increases as a function of the increasing dimensions of the contact surface between the catalyst and the reagents. In order to achieve this, the catalyst must be kept in as divided a state as possible, that is, the solid particles composing it must be as small and individualized as possible. The basic role of the carrier is, therefore, to keep the catalyst particles, or crystallites, in contact with the reagents, in the most divided state possible.
During prolonged use of a catalyst carrier, a reduction of the specific surface area occurs, which is the result of the coalescence of the ultra-fine micropores. During this coalescence, a portion of the catalyst is incorporated into the carrier mass, and may no longer be kept in contact with the reagents.
Up until the present time, most of the ceric oxides that have been prepared give a specific surface area which decreases rapidly under operating temperatures above 500.degree. C. Thus R. ALVERO and colleagues (Journal of the Chemical Society, Dalton Trans. 1984, 87) obtained, using ammonium cerinitrate, a ceric oxide which, after calcination at 600.degree. C., gives a specific surface area of 29 m.sup.2 /g.
Furthermore, Patent No FR-A 2 599 744 contains a description of a ceric oxide which gives a specific surface area of at least 85.+-.5 m.sup.2 /g, obtained following calcination at between 350.degree. and 450.degree. C. and, preferably, between 100 and 130 m.sup.2 /g after calcination at between 400.degree. and 450.degree. C. Said oxide is prepared by means of hydrolysis of watery ceric nitrate solution in a nitric acid medium; the precipate obtained is then separated out, washed with an organic solvent, dried if conditions require, then subjected to calcination. The ceric oxide obtained gives a signficiant specific surface area when prepared within a calcination temperature range of from 300.degree. to 600.degree. C. However, a decrease of the specific surface area after calcination at a higher temperature is observed; the specific surface area is 10 m.sup.2 /g after calcination at 800.degree. C.
The Patent No FR-A 2 559 755 may also be cited. This details a ceric oxide giving a specific surface area of at least 85.+-.5 m.sup.2 /g after calcination at between 350.degree. and 500.degree. C., and is, preferably, between 150 and 180 m.sup.2 /g after calcination at between 400.degree. and 450.degree. C. This oxide is obtained according to a process which involves precipitating out a basic ceric sulfate, by reacting a watery ceric nitrate solution with a watery solution containing sulfate ions, separating out the precipitate thus obtained, washing it using a ammonia hydroxide solution, drying it if necessary, and then calcining it at a temperature varying between 300.degree. and 500.degree. C. The ceric oxide thus obtained gives a large specific surface area, but, when subjected to calcination at 800.degree. C., its specific surface area decreases significantly and falls to about 10 m.sup.2 /g.
In the French Patent Application No 87/09122, the applicant has described a process for increasing and stabilizing at high temperature the specific surface area of a cerc oxide.
This process consists of subjecting ceric hydroxide, precursor of ceric oxide, to solvothermal treatment before calcination.
More particularly, the process described in the abovementioned application involves:
placing the ceric hydroxide in suspension in a liquid medium; PA1 heating it in a closed chamber until a temperature and pressure are achieved which are below the critical temperature and pressure, respectively, of said medium; PA1 cooling the reactive medium and bringing it back to atmospheric pressure; PA1 separating out the ceric hydroxide thus treated; PA1 then calcining it. PA1 placing in suspension in water, or in a watery decomposablebase solution, a ceric hydroxide corresponding to the general formula (I): EQU Ce(OH).sub.x (NO.sub.3).sub.y, pCeO.sub.2, nH.sub.2 O (I) PA1 in which: PA1 heating it in a sealed chamber until a temperature and a pressure are achieved which are less than the critical temperature and pressure, respectively, of said medium; PA1 cooling the reactive medium and bringing it back to atmospheric pressure; PA1 separating out the cericm hydroxide thus treated; PA1 calcining it.
By ceric hydroxide is meant a hydrated ceric oxide CeO.sub.2,2H.sub.2 O, or a ceric hydroxide that may contain residual quantities of bound or adsorbed anions such as chlorides. sulfates, nitrates, acetates, formates, etc.
A preferred embodiment of the process described in the Patent Application No 87/09122 involves use of a stock solution as a liquid medium for autoclaving.
A process of this kind permits not only an increase in the specific surface area of the ceric oxide obtained, but also preservation of an increased specific surface area up to temperatures of 900.degree. C.
By subjecting a ceric hydroxide, prepared by reacting a cerium salt solution with a base, under certain conditions in the presence of an oxydizing agent and with a pH above 7, to treatment by autoclave in a basic medium, the applicant, according to Patent Application FR 87/09122, proposes a ceric oxide giving a specific surface area at 800.degree.-900.degree. C., which has never been achieved by products described according to the state of the technology.
The ceric oxide thus obtained gives a specific surface area of at least 15 m.sup.2 /g, measured after calcination at a temperature of between 800.degree. and 900.degree. C. and, preferably, between 20 and 35 m.sup.2 /g measured after calcination at a temperature of 800.degree. C.
It gives a specific surface area of between 15 and 150 m.sup.2 /g measured after calcination at a temperature of between 350.degree. and 900.degree. C.
In this way, it may give a specific surface area varying between 100 and 150 m.sup.2 /g measured after calcination at between 350.degree. and 450.degree. C.
However, when subjected to a higher temperature of up to 900.degree. C. at the time of use, especially in the area of catalysis, it characteristically retains a specific surface area of at least 15 m.sup.2 /g.
In the present application, the specific surface areas expressed are measured on products which have undergone calcination for at least two (2) hours at the given temperature.
Another characteristic of the ceric oxide, subject of the Patent Application FR 87/09122, is that it gives a porous volume that is above 0.1 cm.sup.3 /g at a measuring temperature of between 800.degree. and 900.degree. C. and that is, preferably above 0.15 cm.sup.3 /g.
The porous volume corresponding to pores having a diameter of less than 60 nm (600 .ANG.) is measured using a mercury porosimeter according to the ASTM D4284-83 standard, or by following the nitrogen-adsorption isotherm method, the abovementioned B.E.T. method.
The porous volume, like the specific surface area, is a function of the calcination temperature: it may vary between 0.35 and 0.15 cm.sup.3 /g for a calcination temperature ranging from 350.degree. to 900.degree. C.
The preferred ceric oxide, subject of the Patent Application 87/09122, gives a porous volume of between 0.15 and 0.25 cm.sup.3 /g after calcination at a temperature of 800.degree. C.
The size of the pores of a calcined ceric oxide at 800.degree. C. ranges between 3 nm (30 .ANG.) and 60 nm (600 .ANG.): the average diameter (d.sub.50) of the pores varies between 20 nm (200 .ANG.) and 30 nm (300 .ANG.), preferably around 25 nm (250 .ANG.).
The definition of the average diameter specifies that all pores of a diameter less than the average make up 50% of the total porous volume (Vp) of the pores having a diameter of less than 60 nm (600 .ANG.).
A ceric oxide calcined at 350.degree. C. gives pores with a diameter of from 2 nm (20 .ANG.) to 100 nm (1000 .ANG.): the average diameter varies between 10 nm (100 .ANG.) and 20 nm (200 .ANG.), and is, preferably, around 15 nm (150 .ANG.).
X-ray diffraction analysis shows that the ceric oxide described in Patent Application FR 878/09122, exhibits a crystalline phase of the CeO.sub.2 having a mesh parameter ranging from 0.542 nm (5.42 .ANG.) to 0.544 nm (5.44 .ANG.). As a guide, it should be specified that the size of the crystallites of a ceric oxide obtained after calcination at 350.degree. C. ranges from 4 nm (40 A) to 6 nm (60 .ANG.) and, after calcination at 800.degree. C., between 10 nm (100 .ANG.) and 20 nm (200 .ANG.).